The present invention relates to the imaging of objects which have been distorted or blurred and more particularly relates to a method and apparatus for creating clear wide field images after first characterizing and then compensating for the distortion. The distortion is characterized by being both spatially and time variant.
The limitations on imaging system performance imposed by a turbulent media, most simply described as xe2x80x98blurring,xe2x80x99 are well known, particularly in applications using medium to large aperture telescopes in the open atmosphere. These limitations have not only led to a variety of system solutions that will be discussed as prior art, but have played a major role in the decision to launch space based telescopes and have led to serious postulations of lunar based observatories.
For a large aperture telescopexe2x80x94generally greater than a 10 centimeter diameter for the visible light regionxe2x80x94which is otherwise constructed to a precision commonly referred to as xe2x80x9cnear diffraction limited,xe2x80x9d the overall ability to resolve objects obscured by a turbulent atmosphere is limited by the turbulence rather than by the instrument. For the visual band of light once more, it is quite typical for a 1 meter aperture telescope to have ten times worse resolving power due to the turbulence, while a 10 meter aperture telescope can be 100 times or more worse than its innate xe2x80x9cdiffraction limit.xe2x80x9d The exact numbers for any given telescope on any given night are a function of many variables, but this general level of degradation is widely recognized. As importantly, this atmospheric blurring directly leads to a loss in effective sensitivity of these large aperture imaging systems, which either renders dim objects just too dim to be seen or forces greatly extended exposure times, ultimately limiting the number of objects that can be imaged during a given length of usage time.
The prior art for addressing this problem and trying to alleviate it can be generally categorized into the following well known areas: 1) Telescope Placement; 2) Adaptive Optics Systems; and 3) Speckle Interferometric Systems. It would not be unfair to say that the system disclosed herein is best summarized as a fundamental expansion to the third category, though this is only in a most general sense.
Regarding the first category, that of simply finding locations of minimal turbulence, the placement of telescopes at high elevations has been known and practiced since Isaac Newton""s era. This typically adds some expense, but more critically, it is well known that this only goes a small way toward reaching diffraction limited performance, and moreover, imaging performance is still quite subject to the varying atmospheric conditions of a given site. The space age has brought about the xe2x80x9cobviousxe2x80x9d solution of launching telescopes above the atmosphere: though at considerable expense and complexity.
The second category of adaptive optics has been well known for decades and has seen significant physical realizations over the last two decades. A brief technical summary of such a system is that after a telescope primary mirror has collected the light waves emanating from a given object, it splits the light wave into two xe2x80x9cbeams.xe2x80x9d One beam goes into an instrument known as a wavefront sensor and the other beam enters an ordinary imaging system or camera. The wavefront sensor derives information about the phase distortion (caused by the atmosphere) of the incoming light wave and in less than hundredths or even thousandths of a second, sends control signals to mirror actuators which advance and retard primary beam mirror surfaces in cancelling opposition to the incoming phase distortion. There are two critical problems with these systems, however. First, they are expensive to build and expensive to maintain. Second, they can only increase the resolving power within an extremely small angle about the nominal central (paraxial) ray of the telescope, typically on the order of 2 to 10 are seconds in the visible band. This xe2x80x9ctunnel visionxe2x80x9d is technically referred to as the xe2x80x9cisoplanatic patch.xe2x80x9d It arises due to the fact that the phase distortion of a three dimensional media such as the atmosphere changes rapidly as a function of field angle.
The third category of speckle interferometry was begun and developed roughly contemporaneously with adaptive optics systems, i.e. over the last two decades. The guiding principle of these systems is to take a large number of short exposure images of an object, thereby xe2x80x9cfreezingxe2x80x9d the phase distortion (in time) of a given single exposure, and ultimately deriving the fourier magnitude and the fourier phase of the object being imaged through mathematical operations on the accumulated set of exposures. The state of the art of these techniques now includes the addition of a wavefront sensor to an overall system, which provides additional information to the computational operations, allowing for a smaller number of short exposures to be gathered to achieve a given level of precision and also simplifying the overall process. Though these speckle systems can be less expensive than adaptive optical systems, they too are limited by xe2x80x9ctunnel visionxe2x80x9d of the isoplanatic patch. Moreover, since they do not account for the field variance of the distortion, they suffer from higher noise and error levels, which in turn affect their effective sensitivity, accuracy, and throughput (number of images needed for a given level of accuracy).
The expense and limited performance of the collective prior art has accordingly limited its application and hampered its broader acceptance. A novel aspect of the system disclosed herein is that it will quantify and compensate for the field variant complex phase distortion across an arbitrarily wide field of view, at a cost much less than a comparable adaptive optics system, and with a throughput and accuracy much better than speckle interferometric systems. In so doing, it can also provide advantages over these prior art systems even in applications where the object of interest is contained within the so-called isoplanatic patch; most notably in cost per image at a given accuracy (error and signal to noise ratio).
Just as adaptive optics systems have recently employed xe2x80x9cartificial beaconsxe2x80x9d to assist in the imaging of very dim objects, so too can the systems described herein utilize various forms of this concept as described herein. Artificial beacons can be employed when the brightness of an object under study is insufficient or inappropriate to provide photons to a wavefront sensor. The beacons are generally laser beams directed along a close line of sight to the object, generating backscatter photons which will undergo largely similar phase distortions as the photons from the object under study, and thus they can be used to deduce the phase distortions applicable to the object photons.
The preferred embodiment of the system disclosed herein is a large telescopic imaging system used in the open atmosphere. This is chosen as an important and tangible example of obtaining clear images through a general field variant distorting media. It should be evident that other physical instruments, subject to distorting mechanisms other than an atmosphere, can also produce sets of digital images and corresponding sets of field variant distortion estimates, thereby being capable of utilizing the claimed methods of processing these sets of digital data into a single clear image.